The present invention relates to a sheet conveyance apparatus for use in a copier; in particular, an automatic sheet conveyance apparatus provided with a sheet reversing device.
Hitherto, there has been provided a sheet conveyance apparatus which is so configured that an original document sheet is led onto sheet glass by means of a conveyor belt that can run across both directions over the sheet glass, then the sheet is conveyed reversely to cause its trailing edge to come in contact with a stepped portion of a stopper plate installed at the end of the sheet glass. In the sheet conveyance apparatus, when feeding a sheet from a sheet supply section, between the conveyor belt and the stepped stopper plate at the sheet glass end, there is provided a gap for sheet passage, while reversely conveying the sheet so as to cause it to come in contact with the stepped stopper plate, the above part is so constituted to make the gap close. In the sheet conveyance apparatus disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter called Japanese Patent O.P.I. Publication) No. 21762-1983, a belt keep roller is supported by a shaft disposing on the moving end of a vertically movable lever which can rock around a fulcrum formed by a drive roller shaft of a conveyor belt, and the moving end of the lever is biased downward so that the belt keep roller is moved upward against the spring when the conveyor belt runs in normal direction and the belt keep roller is moved downward by means of spring force when the conveyor belt runs in reverse direction.
In the above sheet conveyance apparatus, however, its mechanism is complicated, as well as the position of installing the belt keep roller depends indispensably on relative conditions between the drive roller shaft of the conveyor belt and the stepped stopper plate; the above functions cannot be sufficiently achieved, if such position is not set accurately. In addition, if the force of the spring which pushes the belt keep roller downward is too strong, this makes it difficult to provide the gap when the conveyor belt runs in normal direction and, if such force is too weak, this reduces the effect of closing the gap; therefore, adjustment of the spring force is difficult.
Aiming at solving the above problems, there inventors disclosed a new sheet conveyance apparatus in Japanese Patent O.P.I. Publication, No. 229741-1986: Sheet conveyance apparatus is so configured that, a sheet, after it has been led onto sheet glass by means of a conveyor belt capable of running in both directions over the sheet glass, is conveyed reversely to cause its trailing edge to come in contact with a stepped stopper plate provided at the sheet glass end, and is characterized in that a belt keep roller movable interlockingly with the normal and reverse movement of the conveyor belt is provided near the stepped stopper plate.
In the sheet conveyance apparatus, during sheet conveyance by the reverse movement of the conveyor belt, since the belt keep roller continuously presses the stepped stopper plate via the belt, the continuous pressure contact increases the rotating load of the conveyor belt and causes improper rotation; further, the conveyor belt is worn frictionally by the stepped stopper plate, which may result in breakage of the conveyor belt.
As apparatus which can supply efficiently and automatically original sheets to be copied, automated document feeder (hereinafter called the ADF) is available. Hitherto, its function has been limited to copying only one side of a sheet.
In an aspect, lately, a recording apparatus has been developed which is capable of copying both sides of recording paper; this has increasingly required a reversible automatic document feeder (hereinafter called the RADF) that can copy both sides of an original document sheet. Many features that reverse a sheet in the RADF have been proposed. This proposal has been disclosed in Japanese Patent O.P.I. Publication Nos. 216159/1984, 112738/1978 and 8834/1985, Japanese Utility Model O.P.I. Publication No. 4548/1984, and Japanese Patent Application No. 146188/1985.
In such RADF, sheets mounted on a sheet supply tray are sent out one by one by a sheet supply section, and the sent out sheet is fed on sheet glass by the forward running of a conveyor belt and stops at a predetermined position. Then, the side A (obverse) of a sheet is scanned and subjected to light, leading to the formation of copied images on recording paper.
The sheet, whose side A has undergone the copying process, is moved away from the sheet glass by the forward running of the conveyor belt and fed to a paper ejection inversion unit.
Next, when the side B (back) of the sheet is copied, the sheet is inverted in the paper ejection inversion unit and its top end is led again onto the sheet glass. Then, the drive roller has been switched to backward running by the reverse revolution of a reversible motor and the conveyor belt moves the sheet on the sheet glass in the reverse direction. As soon as the top end of the sheet comes in contact with a stepped stopper plate installed at the edge of the sheet glass, the conveyance of the sheet is stopped.
After such stop of sheet conveyance, the side B of the sheet is scanned and exposed to light, resulting in the formation of copied images on recording paper.
After that, the conveyor belt is switched to forward running, and the sheet is conveyed forward on the sheet glass and fed through the paper ejection path of the paper ejection inversion unit via an ejection opening onto the external tray.
In the above proposed RADF, however, the inversion of a sheet for the exposure of the back to light, whose obverse has been exposed to light, is accomplished by means of more than one paper path, resulting in the complicated conveyance. In addition, the conveyor roller must be switched frequently for changing the rotation direction, and many paper supply and ejection selection members must be installed at the paper paths. Thus, such RADF cannot help being large, complicated, and expensive.
In particular, the overall sheet inversion and conveyance path of the inverting section must be much longer than maximum length of a sheet to be fed. So, the inversion section occupies a great volume.
In the conventional sheet conveyance apparatus, a conveyance section is installed above sheet mount glass and, on the elongation of its end, an inverting section is installed in series, resulting in a very large sheet conveyance apparatus and a great installation area.
When opening the sheet mount glass surface in a copier equipped with the conventional large sheet conveyance apparatus having conveyance and inverting sections arranged in series many manual operations may be required. Further, the respective operability in jammed sheet removal and in pressing work for book-like sheets and special sheets is low.
Additionally, to drive and revolve the paper ejection roller and inverting roller of the sheet ejection and inversion unit, the conventional sheet conveyance apparatus transmits power from the drive source directly by using a timing belt or a gear string. Thus, a drive source must be installed independently from a drive source for a conveyor belt; in the result, a great, complicated power transmission system is needed.
In the paper ejection and inversion unit equipped with a paper ejector having a pair of upper and lower rollers coming in pressing contact with each other, a sheet must always be conveyed as being kept in a plane, when the sheet is led to an inversion and circulation passage and is conveyed on the pressed condition by a reversing roller. A sheet to be ejected from the paper ejection roller tends to be jammed.
After a sheet to be ejected from the paper ejection roller onto the paper ejection tray has passed the nipping location of the paper ejection roller, the sheet comes in contact with the tray surface or sheets delivered previously on the tray, is attracted to them, and then is not ejected to a correct position. Thus, the trailing edge of the sheet stays at the passage near the paper ejection roller and causes a following sheet to be jammed.
Hitherto, there has been proposed a method of providing ribs on the paper ejection roller to cause a fed sheet to form a curved wave-like surface. In the RADF which is equipped with a common passage used for a paper ejection path and an inversion and circulation path at the downstream side of the nipping location of the paper ejection rollers having the ribs thereon, paper may be ejected successfully, but when a sheet is inverted and circulated, the curved wave-like surface tends to be rumpled by the inversion nipping roller, damaging the sheet.
A sheet conveyed directly by the conveyor belt in the RADF is moved with a relative friction onto a sheet glass surface, thereby being electrostatically charged. The sheet introduced into the paper ejection and inversion unit of the RADF comes in contact with guide plates forming the conveyance path and further produces electrostatic charges frictionally. The higher the sheet conveyance speed, the more frictional electrostatic charges a sheet produces. In particular, during automatic sheet inversion and conveyance in which sheets are repeatedly fed in normal and reverse conveyance direction, the amount of produced frictional electrostatic charges tends to be large.
When a sheet fed from the sheet inversion section of the above paper ejection and inversion unit onto the sheet glass surface is further conveyed by being inserted between the conveyor belt and the sheet glass, if the sheet is electrostatically charged, the charged sheet is repulsed from or adsorbed by the sheet glass or the conveyor belt, resulting in positional deviation and deformation, and thus further causing improper conveyance. So, a sheet is not set properly or broken at its edge. The result is that a sheet is not copied suitably, is damaged, or jammed.